William B. McKinnon

Convective instability in Europa's floating ice shell

Models of the tidally heated, floating ice shell proposed for the jovian satellite Europa generally find shell thicknesses less than 30 km. Past parameterized convection models indicated that such shells are stable against convective overturn, which otherwise ostensibly leads to freezing of the ocean underneath. Here I apply the temperature-dependent viscosity convection scaling developed by Solomatov and coworkers to the Europan ice shell. The temperature-dependent properties of ice are linearized about 260 K, as any convective interior should be close to this temperature, with the colder ice forming an essentially passive, stagnant lid. Ice shells ≳ 10 km thick are found to be unstable to convection at their base for melting-point viscosities of 1013 Pa-s (as linearized by tidal stresses), if the ice deforms by superplastic creep, but such low viscosities require small grain sizes (<1 mm). This requirement may be met if grain sizes observed in terrestrial polar glaciers can be strain-rate scaled to Europa. Regardless, convection at the base of the ice shell, if initiated, may not freeze the ocean. Because of tidal heating, a stagnant-lid regime ice shell is much more dissipative than a conductive shell of the same thickness. Such a shell should thin, not thicken, and the potential exists for further thermal instabilities and runaways.

Fault offsets and lateral crustal movement on Europa - Evidence for a mobile ice shell

Abstract Right-lateral structural offsets of ∼25 km have been identified on the icy Galilean satellite Europa. These occur along dark lineaments oriented orthogonally to wedge-shaped bands, which are also ∼25 km wide. Wedge-shaped bands are interpreted as dilated tension fractures, which formed as crustal blocks (or plates) 50–100 km across separated and slipped past each other along flanking strike-slip faults. This style of deformation does not appear to be characteristic of other lineament types, and with the exception of Earth appears to be unique to Europa. Together, the subparallel wedge-shaped b bands form a broad NW-SE trending belt, ∼1500 km long and less than 500 km across, near the anti-Jovian point. This belt is interpreted as a major crustal fracture (or rift) zone, with a pole of rotation (determined by the strike-slip faults) near 47°S, 144°W, and an approximate NE-SW direction of maximum tensile stress. Extension may have been areally compensated at Agenor Linea, a bright band of possible compressional origin. Global expansion, tidal distortion, and nonsynchronous rotation do not explain the inferred minimum principal (i.e., least compressive) stress directions. Alternatively, fracturing near the anti-Jovian point may be a result of (i) solid-state convection in the lower ice crust (possibly triggered by uneven heat flow from the silicate interior), (ii) rotation of the icy shell about the sub- and anti-Jovian points induced by latitudinal lithospheric thickness variations, or (iii) preferential strain accumulation from the rest of the icy shell. No significant distortion of the crustal blocks occurred during fracturing and rotation, indicating that the icy crust was probably mechanically decoupled from the silicate interior in this region over the time scale of fracturing. Decoupling on a global scale is also likely the simple geometry of lineaments argues for formation in an icy lithosphere, not a silicate one, and other evidence for fracturing caused by nonsynchronous rotation stress is not compatible with a tidally locked silicate interior (which we show is likely) unless the ice shell rotates independently. Decoupling could have been due to either warm ice or liquid water near the base of the icy crust. Mechanical bounds on lithospheric thickness (a few to ∼10 km) lead to heat flow estimates that admit both possibilities but favor decoupling by liquid water.

Three-layered models of Ganymede and Callisto: compositions, structures, and aspects of evolution

Abstract Three-layered structural models are determined for Ganymede and Callisto. Each consists of a rock core, a mixed ice-rock lower mantle, and a pure ice upper mantle. This structure results from differentiation subsequent to accretional melting. Attention is given to evaluating various candidates for the rock component and three alternatives, representing various degrees of silicate hydration and oxidation, are modeled and incorporated. Structures are calculated on the basis of a 250°K isotherm, which is a reasonablev approximation to the gentle adiabats expected to occur in icy satellites. Differentiation of an ice-rock satellite generally involves an increase in radius, and the three-layered approach allows this process to be examined in some detail. It is determined that satellite expansion is most significant early in the process and much less so as differentiation proceeds to completion. If tectonics are due to global expansion, distinguishing on this basis between a completely differentiated satellite and one that is only partially differentiated is difficult. The postaccretional global expansion of Ganymede, which may have left a tectonic record, was probably limited to 1% in radius, in agreement with observed limits. Useful quantities such as silicate mass and volume fraction, uncompressed density, J2, C22, binding energy, and surface heat flow are also determined. Nonhydrostatic contributions to J2 and C22 are estimated and shown to be nonnegligible. Encounters with Jupiter-orbiting spacecraft are unlikely to determine Callistos degree of central condensation. We conclude by calculating the relative likelihood of postaccretional melting caused by radiogenic heating. Three-layered satellites have generally hotter interiors, because additional thermal boundary layers divide the separately convecting upper and lower mantles, inhibiting heat transport. Ganymedes and Callistos that are less than about 1 3 differentiated (by mass) should experience a second episode of melting, as these boundary layers are either above the level of the water-ice minimum-melting temperature or intersect the melting curve at deeper levels. Runaway differentiation to at least a depth corresponding to a pressure in the ice V stability field is likely. The main point here is that if satellite tectonics are tied to differentiation by melting or its aftermath (as in the instability following ocean closure of Kirk and Stevenson), moderate or small amounts of accretional differentiation are unlikely to explain an absence of tectonics (as in Callisto), because extensive differentiation ultimately occurs.

The geology of Pluto and Charon through the eyes of New Horizons

New Horizons unveils the Pluto system In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanitys first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H2O, CH4, CO, N2, and NH3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come. Science, this issue pp. 1284, 10.1126/science.aad9189, 10.1126/science.aad8866, 10.1126/science.aae0030, & 10.1126/science.aad9045 Pluto and Charon display a complex geology, including evidence for tectonics and cryovolcanoes. NASA’s New Horizons spacecraft has revealed the complex geology of Pluto and Charon. Pluto’s encounter hemisphere shows ongoing surface geological activity centered on a vast basin containing a thick layer of volatile ices that appears to be involved in convection and advection, with a crater retention age no greater than ~10 million years. Surrounding terrains show active glacial flow, apparent transport and rotation of large buoyant water-ice crustal blocks, and pitting, the latter likely caused by sublimation erosion and/or collapse. More enigmatic features include tall mounds with central depressions that are conceivably cryovolcanic and ridges with complex bladed textures. Pluto also has ancient cratered terrains up to ~4 billion years old that are extensionally faulted and extensively mantled and perhaps eroded by glacial or other processes. Charon does not appear to be currently active, but experienced major extensional tectonism and resurfacing (probably cryovolcanic) nearly 4 billion years ago. Impact crater populations on Pluto and Charon are not consistent with the steepest impactor size-frequency distributions proposed for the Kuiper belt.

On the Origin of Triton and Pluto

Lyttleton hypothesized long ago that Triton and Pluto originated as adjacent prograde satellites of Neptune1. With the presently accepted masses of Triton and Pluto–Charon2,3, however, the momentum and energy exchange that would be required to set Triton on a retrograde trajectory is impossible. The mass of Triton has probably been seriously overestimated4,5, but not by enough to relax this restriction. It is implausible that the present angular momentum state of Pluto–Charon has been significantly influenced by Neptune6. It could not acquire such angular momentum during an ejection event unless a physical collision was involved, which is quite unlikely. The simplest hypothesis is that Triton and Pluto are independent representatives of large outer Solar System planetesimals. Triton is simply captured, with potentially spectacular consequences that include runaway melting of interior ices and release to the surface of clathrated CH4, CO and N2 (ref. 7). Condensed remnants of this proto-atmosphere could account for features in Tritons unique spectrum8–11.

Geodynamics of Icy Satellites

Geodynamics concerns the internal structure, differentiation and convection, and tectonics of worlds. With respect to icy satellites there exists an excellent literature (e.g., Burns, 1986), and for the Earth a formidable body of new research results. In this review, I update some of the perspectives on how the icy satellites operate geodynamically, addressing the interplay between rheology, petrology, convection, and tectonics, and focusing on convection as a predominant endogenic process. Icy satellites, if they do undergo internal convection, are generally in the stagnant lid regime as defined by Solomatov, because the viscosity of water ice is strongly temperature-dependent. The Rayleigh number, a measure of the vigor of convection, for the actively convecting interior of an icy satellite is a very strong function of satellite radius (going at least as the sixth power). Convection was probable (if not vigorous) in all but the smallest middle-sized icy satellites early in solar system history. Today, vigorous convection only occurs in Ganymede, Callisto, and Titan, with weak convection occurring in Triton and Pluto. The pronounced polymorphism of the predominant ice, water ice, is expected to strongly modulate convective flow. The ice I-to-II transition should augment convective vigor, while both the ice I-to-III and II-to-V transitions should, by themselves, inhibit convective penetration. Convection within the larger icy satellites should be or have been layered. The negative activation volume for ice I ensures that convective flow in ice I is strongly coupled to the overlying icy lithosphere, which may in some circumstances generate sufficient stress in the lithosphere to induce brittle failure and surface tectonics.

On the origin of the Pluto-Charon binary

The normalized angular momentum density of Pluto-Charon (0.45) exceeds the critical value of 0.39 above which no stably rotating single object exists, suggesting a collisional origin for this binary. The effects of viscosity on Plutos rotational stability and on the density of Charon are considered. Both a more or less dense Charon would be consistent with a collisional origin if one (the least massive) or both protoobjects were differentiated. It is noted that the angular momentum of the system requires the protoobjects to be comparably (if not equally) sized if off-center impact velocities vary between escape (about 1.3 km/s) and somewhat greater values (about 2.5 km/s) appropriate to Plutos eccentric and inclined solar orbit. 30 refs.

Flooding of Ganymede's bright terrains by low-viscosity water-ice lavas

Large regions of the jovian moon Ganymede have been resurfaced, but the means has been unclear. Suggestions have ranged from volcanic eruptions of liquid water or solid ice to tectonic deformation, but definitive high-resolution morphological evidence has been lacking. Here we report digital elevation models of parts of the surface of Ganymede, derived from stereo pairs combining data from the Voyager and Galileo spacecraft, which reveal bright, smooth terrains that lie at roughly constant elevations 100 to 1,000 metres below the surrounding rougher terrains. These topographic data, together with new images that show fine-scale embayment and burial of older features, indicate that the smooth terrains were formed by flooding of shallow structural troughs by low-viscosity water-ice lavas. The oldest and most deformed areas (the ‘reticulate’ terrains) in general have the highest relative elevations, whereas units of the most common resurfaced type—the grooved terrain—lie at elevations between those of the smooth and reticulate terrains. Bright terrain, which accounts for some two-thirds of the surface, probably results from a continuum of processes, including crustal rifting, shallow flooding and groove formation. Volcanism plays an integral role in these processes, and is consistent with partial melting of Ganymedes interior.

Effect of Enceladus's rapid synchronous spin on interpretation of Cassini gravity

Enceladuss degree 2 gravity, determined by Cassini, is nominally nonhydrostatic to 3σ (J2/C22 = 3.38–3.63, as opposed to 10/3). Iess et al. (2014) interpret this in terms of a hydrostatic interior (core) and isostatic (not hydrostatic) floating ice shell. Enceladuss rapid (1.37 d) synchronous spin and tide distorts its shape substantially, though, enough that the predicted hydrostatic J2/C22 is not 10/3 but closer to 3.25. This leads to the following revision to the internal picture of Enceladus, compared with Iess et al.: (1) the satellites core is somewhat smaller and slightly denser (190 km radius and 2450 kg/m3); (2) the compensation depth (shell thickness) of the global (degree 2) ice shell is ≈ 50 km, rather close to the base of the modeled ice + water layer; and (3) the compensation depth (shell thickness) beneath the South Polar Terrain (from J3) remains shallower (thinner) at ≈ 30 km, independent of but influenced by the degree 2 solution.

New Horizons: Anticipated Scientific Investigations at the Pluto System

The New Horizons spacecraft will achieve a wide range of measurement objectives at the Pluto system, including color and panchromatic maps, 1.25–2.50 micron spectral images for studying surface compositions, and measurements of Pluto’s atmosphere (temperatures, composition, hazes, and the escape rate). Additional measurement objectives include topography, surface temperatures, and the solar wind interaction. The fulfillment of these measurement objectives will broaden our understanding of the Pluto system, such as the origin of the Pluto system, the processes operating on the surface, the volatile transport cycle, and the energetics and chemistry of the atmosphere. The mission, payload, and strawman observing sequences have been designed to achieve the NASA-specified measurement objectives and maximize the science return. The planned observations at the Pluto system will extend our knowledge of other objects formed by giant impact (such as the Earth–moon), other objects formed in the outer solar system (such as comets and other icy dwarf planets), other bodies with surfaces in vapor-pressure equilibrium (such as Triton and Mars), and other bodies with N2:CH4 atmospheres (such as Titan, Triton, and the early Earth).